Formation of Scorpius X-1 induced by anomalous magnetic braking of Ap/Bp stars

Abstract

Sco X-1 is the brightest persistent X-ray in the sky. It is generally believed that Sco X-1 is a low-mass X-ray binary containing a neutron star accreting from a low-mass donor star where mass transfer is driven by the magnetic braking. However, the mass transfer rate predicted by the standard magnetic braking model is at least one order of magnitude lower than the one inferred by X-ray luminosity. In this work, we investigate whether this source could evolved from an intermediate-mass X-ray binary including Ap/Bp stars with a slightly strong magnetic field of 300 - 1000 G. The coupling between the magnetic field and an irradiation-driven wind induced by the X-ray flux from the accretor can yield a strong magnetic braking, which could give rise to a relatively high mass transfer rate. According to the observed orbital period, the mass transfer rate, the mass ratio, and the donor star spectral type, the progenitor of Sco X-1 should be an intermediate-mass X-ray binary including a 1.6 $-$ 1.8 $\rm M_{\odot}$ Ap/Bp donor star in a 1.3 $-$ 1.5 day orbit. Therefore, we propose that anomalous magnetic braking of Ap/Bp stars provides an alternative evolutionary channel to a part of luminous X-ray sources.